Method and device for design of preform

ABSTRACT

On the basis of the controllability of wall thickness distribution of a mold product by a preform shape, the number of fabrication steps and the cost required for a design and trial formation of a preform forming metal mold are reduced by designing a preform shape such that a thickness distribution of the preform leads a wall thickness distribution of an aimed mold product to thereby obtain a mold product having an ideal rigidity and stability thereof.

TECHNICAL FIELD

The present invention relates to a designing of a plastic mold product,particularly to a plastic molding method by using an injection blowmolding in which a preform is put in a blow metal mold and blowing gasinto the metal mold to expand the preform. The present invention isapplicable to a designing of a metal mold for a plastic molding.

BACKGROUND ART

Blow molding of a plastic mold product using a metal mold is roughlyclassified to:

-   1) extrusion blow molding; and-   2) injection blow molding.

The injection blow molding can be further classified to:

-   2a) non-stretch injection blow molding; and-   2b) stretch blow molding.

FIG. 13A to FIG. 13C show procedures in the respective blow moldingmethods. FIG. 13A shows the procedures of the extrusion blow moldingmethod, FIG. 13B shows the procedures of the non-stretch blow moldingmethod and FIG. 13C shows the procedures of the stretch blow moldingmethod.

The extrusion blow molding fabricates a product having an outer shapecorresponding to an inner shape of, for example, a metal mold byextruding molten plastic material into an interior of the metal mold,locking the metal mold and blowing gas into the molten plastic materialto urge the latter onto an inner wall of the metal mold. On the otherhand, the injection blow molding uses a preliminarily prepared preform.The preform is heated or temperature-regulated on demand, confined in ablow metal mold and expanded by gas blow. The non-stretch blow moldingdiffers from the stretch blow molding in that the non-stretch blowmolding to the preform is performed by using only air while the stretchblow molding to the preform is performed by stretching the preform in alongitudinal direction by means of a stretch rod and then stretching itin a lateral direction by blow air. The stretch blow molding issometimes called as “biaxial stretch blow molding”.

Japanese Patent Nos. 2955509 and 2957503 (both assigned to the assigneeof this application) disclose methods for designing a metal mold in theblow molding by predicting a thermal deformation of a product after ametal mold is stripped from the product. By utilizing this method, it ispossible to design a metal mold capable of obtaining a mold producthaving an aimed shape even if a cooling time thereof in the metal moldis shortened and the producibility is improved by realizing a reductionof molding time by the metal mold. Further, JP2001-322160A discloses amethod for reasonably designing a die and a core for extruding a parisonby simulating a shape of the parison in an extrusion blow molding.

In the injection blow molding, a preform is formed by injection molding.Therefore, the injection blow molding is advantageous in that it ispossible to design a parison shape including wall thickness thereofprecisely compared with a parison formed by the extrusion blow molding.On the other hand, wall thickness of a mold product passed through ablow molding in a later step is generally unpredictable. If a differenceof the wall thickness of the mold product from a designed value islarge, rigidity of the mold product is lowered. Such difference of wallthickness of the mold product may cause a considerable deformation ofthe mold product particularly when a metal mod is stripped from the moldproduct at high temperature after the blow molding, resulting in aproblem of degradation of the stability of the mold product. In order toobtain an ideal wall thickness distribution for obtaining requiredrigidity and stability of a mold product, it is usually necessary tochange a shape of a preform in a trial and error manner. Therefore,there is a problem that a designing and trial forming of preform(designing and trial forming of a metal mold for preform molding) in theinjection blow molding is time consuming. Further, temperatureregulation of preform before blow molding is important. That is, sinceproperties of resin such as viscosity of molten plastics areconsiderably changed by temperature, the temperature regulation causesthe designing and trial forming of preform to be time consuming.

The above problems become more conspicuous in the stretch blow moldingin which a stretching is performed when the blow molding is performed.

A conventional method for designing a preform will be described withreference to FIG. 14. In the injection blow molding, lateral stretchrate, longitudinal stretch rate and area stretch rate=(surface area ofmold product)/(surface area of preform) strongly influence themoldability. Since a mouth portion of a preform usually becomes a mouthportion of an aimed mold product (bottle) as it is, a diameter of thepreform is determined by a diameter of the mouth portion of the aimedmold product. A standard of longitudinal stretch rate is 1˜1.2 times inthe case of non-stretch blow molding and 1.5˜3 times in the case ofstretch blow molding, although it depends upon resin used. A standard ofwall thickness of a preform is about 3 mm (2˜4 mm). When the wallthickness is small, the molding of the preform becomes difficult. On theother hand, when the wall thickness is large, the whole molding cyclebecomes long and the blowing step becomes difficult. The wall thicknessof the preform is determined by the longitudinal stretch rate and theaimed wall thickness after molded.

An inner diameter of the preform is determined on the basis of an innerdiameter of the mouth portion of the preform. However, a certain draftangle for stripping an inner metal mold from the preform is practicallynecessary. Therefore, a practical preform has such shape as shown inFIG. 15. The wall thickness may be finely regulated as shown in FIG. 16on the basis of resin material to be used and rigidity specification ofthe aimed mold product. Since an outer shape thereof is changed bychanging the wall thickness distribution, the design of wall thicknessdistribution and the design of shape have the same meaning.Conventionally, however, the design of wall thickness distribution of apreform is performed empirically and appropriateness of wall thicknessdistribution of a mold product can be known after the product ispractically molded.

A technique for determining a range of a height or a range of an averagediameter of a preform is proposed in JPH7-108595A, in which the range ofa height h and the range of an average outer diameter d of the preformis determined by using the finite-element method as the preform designmethod, performing a simulation with using the height h and the averageouter diameter of a preform as design variables, sampling maximum andminimum wall thickness values of a mold product obtained as a result ofsimulation and determining whether or not the maximum and minimumthickness values are within a tolerable range. In this technique,however, the height h and the average diameter d are merely changed.Therefore, when the mold product is, for example, a bottle, it isimpossible to increase wall thickness of a substantially stretchedportion of the bolt such as a shoulder portion thereof, so that anoptimal shape of the preform can not be obtained. Further, sinceoptimization of the height h and optimization of the average diameter dare performed independently, it is not always possible to obtain anoptimal solution.

DISCLOSURE OF INVENTION

The present invention is intended to solve such problems in theinjection blow molding and an object of the present invention is toprovide a preform designing method and a trial fabrication apparatustherefor with which time, forming step number and cost required for thedesigning and trial formation of an ideal shape of a preform can bereduced. Another object of the present invention is to provide a preformdesigning method and a trial forming apparatus therefor with which time,forming step number and cost required for the designing and trialformation of a metal mold for an optimal preform can be reduced.Particularly, an object of the present invention is to provide adesigning method and an apparatus with which an optimal preform shapecan be obtained.

In the injection blow molding, a preform is molded by a preform moldingmetal mold. The preform thus molded is put in a blow molding metal moldand molded to a mold product by blow molding. The present invention isused in designing the preform molding metal mold.

The preform designing method according to the present invention isfeatured by comprising the steps of calculating a wall thicknessdistribution of a blow mold product by performing a blow simulation fora shape of a preform including wall thickness distribution thereof,comparing the wall thickness distribution of the product calculated fromthe simulation with a wall thickness distribution of an aimed moldproduct, changing the shape of the preform on the basis of a differencetherebetween and repeating the calculating step, the comparing step andthe changing step until the difference becomes within a threshold valuerange.

Since, in the present invention, the simulation is performed byrepeatedly changing the wall thickness distribution of the preform untilthe difference between the aimed wall thickness of a mold product andthe calculated wall thickness distribution of the mold product becomesequal to or smaller than the threshold value, it is possible to obtainan optimal shape of the preform. Further, since the calculated wallthickness of the product is compared with the aimed wall thickness ofthe product at every node, it is possible to obtain an optimal shape ofthe preform.

That is, according to a first aspect of the present invention, thepreform designing method is featured by comprising a first step ofcalculating wall thickness of a mold product on the basis of an initialshape of a preform, a second step of obtaining a difference between thecalculated wall thickness distribution of the mold product and an aimedwall thickness distribution, a third step of comparing the differencewith a threshold value, a fourth step of changing the initial shape ofthe preform on the basis of the difference when the difference exceedsthe threshold value and a fifth step of repeating the first to fourthsteps until the difference becomes smaller than the threshold value.

Incidentally, it is possible to preliminarily set an aimed wallthickness distribution of a mold product, to execute a blow simulationby inputting a wall thickness distribution at discrete nodes of apreform such that a total weight of the preform coincides with a totalweight of the aimed mold product, to compare the wall thickness obtainedat every node of the calculated mold product with the aimed thickness atpositions corresponding to the nodes, to change the wall thicknessdistribution of the preform on the basis of the difference obtained bythe comparison and to repeat the above procedures until the differenceat every node becomes smaller than a threshold value. The preliminarysetting of the aimed wall thickness distribution of the mold product maybe performed by setting discrete points in the aimed mold product andassigning wall thickness to the discrete points. In general, thediscrete points of the aimed mold product do not coincide with the nodesof the calculated mold product, which are obtained by the simulation.The nodes of the preform are obtained from discretization of an innershape of the preform and the outer shape of the preform is determined bygiving wall thickness outside of the nodes.

Further, it is preferable to calculate the wall thickness distributionof the mold product by using a temperature distribution of the preform.

According to a second aspect of the present invention, a preformdesigning apparatus is provided, which is featured by comprising meansfor storing an input initial shape of a preform as a three dimensionalinformation, first means for calculating a wall thickness distributionof a mold product from the initial shape of the preform, second meansfor calculating a difference between the calculated wall thicknessdistribution of the mold product and a wall thickness distribution of anaimed mold product, third means for comparing the difference with athreshold value, fourth means for changing the wall thicknessdistribution of the preform on the basis of the difference when thedifference exceeds the threshold value and fifth means for repeatedlyexecuting the first to fourth means until the difference becomes smallerthan the threshold value.

Incidentally, the storing means preliminarily stores an information ofan aimed wall thickness distribution of the mold product, the firstmeans includes means for performing a blow simulation in response towall thickness distribution at discrete nodes of the preform such that atotal weight of the preform coincides with that of the aimed moldproduct and the second means may include means for comparing wallthickness at the node of the thus calculated mold product with the aimedwall thickness at a position corresponding to the node.

According to a third aspect of the present invention, a program capableof executing the above mentioned respective means by a computer systemhaving a basic software and the program installed therein is provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a main portion of a design apparatus fordesigning a preform for use in a product molding according to anembodiment of the present invention;

FIG. 2 illustrates a designing method for designing a shape of apreform, according to the present invention;

FIG. 3 is a flowchart showing a preform shape designing procedure of thepresent invention;

FIG. 4 is a flowchart showing a temperature regulating condition settingprocedure of the present invention;

FIG. 5 is a flowchart showing a blow metal mold designing procedure;

FIG. 6 is a cross sectional shape of a preform;

FIG. 7 shows an inside shape of a preform;

FIG. 8 shows an inside shape of a mold product;

FIG. 9 shows a wall thickness designing procedure with respect to a nodei;

FIG. 10 shows a preform heating model;

FIG. 11 shows an outer diameter of a preform and a distance of an aimedposition from a center axis;

FIG. 12 illustrates a concept of intensity of radiation;

FIG. 13A, FIG. 13B and FIG. 13C illustrate an extrusion blow method, annon-stretch injection blow method and a stretch blow method,respectively;

FIG. 14 illustrates a stretching rate;

FIG. 15 shows a cross sectional shape of a preform having a draft angle;and

FIG. 16 shows a cross sectional shape of a preform having regulated wallthickness.

BEST MODE FOR CARRYING OUT THE INVENTION

A designing apparatus of a preform according to an embodiment of thepresent invention will be described with reference to FIG. 1 to FIG. 5,in which FIG. 1 is a block diagram of the preform designing apparatus,FIG. 2 shows a preform design procedure, FIG. 3 is a flowchart showing adesigning procedure of a shape of a preform, FIG. 4 is a flowchartshowing a condition setting procedure in a temperature regulation stepand FIG. 5 is a flowchart showing a blow metal mold designing procedure.

As shown in FIG. 1, the preform designing apparatus according to thisembodiment comprises an input portion 1 for inputting an initial shapeof a preform including wall thickness distribution as a threedimensional information, a simulation execution portion 2 forcalculating a deformation simulation (blow simulation) for a deformationof the preform when the latter is put in a metal mold and blown by gas,a difference calculation portion 3 for calculating a difference in wallthickness distribution between a mold product resulting from thedeformation simulation and an aimed mold product, a comparison portion 4for comparing the difference with a threshold value and a designchanging portion 5 for changing the wall thickness distribution of thepreform on the basis of the difference when the difference exceeds thethreshold value, and is featured by repeatedly executing the simulationexecution portion 2, the difference calculation portion 3, thecomparison portion 4 and the design changing portion 5 until thedifference becomes smaller than the threshold value.

The preform designing apparatus of this embodiment utilizes a computerdevice and is realized by installing a program corresponding to anoperation of the preform designing apparatus in the computer device inwhich a basic software (operating system (OS)) is installed. The programmay be recorded on a recording medium and installed by using therecording medium. Alternatively, the program may be installed through acommunication line.

Now, an operation of the preform designing apparatus of this embodimentwill be described along with an injection blow molding method. In thepreform designing method of the present invention, the operation startsfrom a preform shape having a wall thickness distribution and isfeatured by that the number of steps for designing the preform metalmold shape is made irreducibly minimum and an optimal preform shape isobtained by comparing a wall thickness distribution of a calculated moldproduct with, a wall thickness distribution of an aimed mold product,changing the preform shape (wall thickness distribution) on the basis ofa difference therebetween and repeating these operations to converge thedifference.

As shown in FIG. 2 showing the design procedure, a preform shape (wallthickness distribution) design is performed by blow analysis after atemperature distribution of a preform is given and a preform heatingcondition and a preform cooling condition under which a temperaturedistribution similar to the set temperature distribution of the preformis obtained are obtained by a heating-cooling analysis. The blowanalysis of the preform is performed again with the preform temperaturedistribution obtained by the heating condition and the cooling conditionthus set. When a change of the metal mold for molding the preformbecomes necessary, a change of the metal mold for preform is performed.Incidentally, it may be possible to obtain the preform temperaturedistribution by experiments without performing the heating-coolinganalysis. In the blow analysis, it is necessary to give materialproperties by taking the preform temperature distribution intoconsideration. It is preferable that the analysis is performed by usinga temperature dependency of the material properties of the preform andconsidering a temperature change with time.

That is, as shown in FIG. 1, the three dimensional information of theinitial shape (wall thickness distribution) of the preform and aninitial setting value of the preform temperature distribution aresupplied to the input portion 1 and stored therein. The threedimensional information and the initial setting value are taken in thesimulation execution portion 2 from the input portion 1. In thesimulation execution portion 1, the blow analysis shown in FIG. 3 isperformed by using these parameters. A result of the analysis isinputted to the difference calculation portion 3 in which the aimedshape of the mold product and the wall thickness distribution thereofare calculated. The difference is compared with the threshold value inthe comparison portion 4 and, when the difference is larger than thethreshold value, the preform shape (wall thickness distribution) ischanged on the basis of the difference in the design change portion 5.

Now, how to determine the shape of the preform will be described withreference to FIG. 6, which shows a cross sectional shape of the preform.First, an inside shape of the preform is determined by the innerdiameter of a mouth portion of an aimed mold product and the draft angleat discrete nodes. An outer shape of the preform is obtained bythickening the respective nodes. The blow simulation is performed forthis preform shape. Incidentally, the inner diameter of the preform isthe same as that of the mouth portion of the aimed mold product(bottle). Although the longitudinal stretch rate depends upon the resinmaterial used, standards are 1˜1.2 times in the case of non-stretch(height of the preform is about 0.8˜1 times that of the aimed moldproduct) and 1.5˜3 times in the case of stretch blow (height of thepreform is about 0.3˜0.7 times that of the aimed mold product). Thethickness of the preform may be about 3 mm as in the conventional case.However, in the present invention, when the shape and rigidityspecification of the aimed mold product is substantially the same asthat of a mold product, the preform of which was designed previously, itis possible to set the initial shape of the preform by referring to thestored design data (inner diameter, shape and thickness of the preform)and start the designing (calculation). To give a wall thicknessdistribution is to give the shape of the preform.

The temperature distribution is changed depending upon the heatingcondition. As shown in FIG. 4, the temperature distribution of thepreform is determined by inputting the shape (wall thicknessdistribution), heater power and an arrangement of heat shieldingconstruction to the simulation execution portion 2 through the inputportion 1. The term “heat shielding construction” represents means,which is provided between a heater and the preform for shielding thermalradiation from the heater and utilized to regulate the temperaturedistribution. In this embodiment, the heat shielding means is aplurality of aluminum rods disclosed in JPH11-207806A. In the simulationexecution portion 2, the heating-cooling analysis is performed tocalculate the temperature distribution of the preform. The temperaturedistribution of the preform is inputted to the comparison portion 4(dotted line in FIG. 1) and compared with the set temperaturedistribution. The heater power and the arrangement of the heat shieldingmeans are changed according to a result of the comparison.

The change of the shape of the preform (wall thickness distribution) andthe setting of the heating condition are performed in this manner. Inthe present invention, when these changes are not enough, the shape ofthe blow metal mold is changed along the flowchart shown in FIG. 2according to the methods disclosed in Japanese Patent Nos. 2955509 and2957503. That is, as shown in FIG. 5, when the wall thicknessdistribution of the mold product is calculated from the shape of thepreform by the blow analysis, the simulation execution portion 2performs the cooling analysis and the thermal deformation analysis onthe basis of the calculated wall thickness distribution to calculate theshape of the mold product after the blow metal mold is stripped from themold product to thereby obtain a difference from the aimed shape. In thedesign changing portion 5, the shape of the blow metal mold is changedon the basis of the difference.

A concrete example of the wall thickness distribution designing of thepreform will be described with reference to FIG. 7, which shows aninside shape of a preform, and FIG. 8, which shows an inner shape of themold product. Describing the wall thickness design procedure withreference to a node i, the blow analysis is performed at k-thcalculation according to a wall thickness design value Hi(k) of thepreform at the node i to obtain a mold product wall thickness hi(k) atthe node i and a difference Δhi(k) between thickness hi(k) and an aimedwall thickness hi^(target) is obtained by Δhi(k)=hi(k)−hi^(target), asshown in FIG. 9. Thus, in (k+1)th calculation, the wall thickness of thepreform at the node i is updated asHi(k+1)=Hi(k)−α′Δhi(k)where □′=□(Haverage/haverage) where □ is converging accelerationcoefficient, Haverage is an average wall thickness of the preform andhaverage is an average wall thickness of the mold product. Further, thewall thickness Hi(k+1) is corrected such that the total weight of thepreform coincides with the total weight of the aimed mold product. Thecalculation is repeated until the difference Δhi(k) becomes equal to orsmaller than the threshold value. The threshold value it usually usedfor all nodes commonly. However, in order to perform precise design forthe mechanical strength such as bucking strength of a portion of themold product to which higher stress is concentrated than other portions,different threshold values may be used for the respective nodes.

Now, an example of the preform thermal analysis will be described. Asshown in FIG. 10, a preform heating model is set. In the example shownin FIG. 10, the preform is a circular cylinder, the heater isrepresented by a line having infinite length and a reflection plate isprovided on a floor. Thermal radiation received by a unit surface areaof the preform in a unit time is calculated by using this model.

Radiation heat E0 received by a unit surface area of a preform in a unittime is a sum of direct radiation heat E^(dir) received from a heaterdirectly and indirect radiation heat E^(ref) received from a reflectionplate in a unit area and in a unit time and E^(dir) and E^(ref) arerepresented by the following equations, respectively: $\begin{matrix}{{E^{dir}\left( {\theta,z} \right)} = {\frac{\cos\quad\theta}{2\pi}{\sum\limits_{n = 1}^{N}{\frac{X_{n} - x}{R_{n}^{2}}I_{n}}}}} \\{where} \\{{R_{n} \equiv \sqrt{\left( {X_{n} - x} \right)^{2} + \left( {Z_{n} - z} \right)^{2}}},{x \equiv {R\quad\cos\quad\theta}}} \\{{E^{ref}\left( {\theta,z} \right)} = {\frac{I_{cor}\quad\cos\quad\theta}{2\pi}{\sum\limits_{n = 1}^{N}{\frac{X_{n} - X_{r}}{\sqrt{\left( {X_{n} - X_{r}} \right)^{2} + \left( {Z_{n} - Z_{r}} \right)^{2}}}\frac{I_{n}}{R_{n}^{ref}}}}}} \\{{where}\quad} \\{{X_{r} \equiv \frac{{x\left( {Z_{n} - Z_{r}} \right)} + {X_{n}\left( {z - Z_{r}} \right)}}{\left( {z - Z_{r}} \right) + \left( {Z_{n} - Z_{r}} \right)}},{R_{n}^{ref} \equiv \sqrt{\left( {X_{n} - x} \right)^{2} + \left( {Z_{n} + z - {2Z_{r}}} \right)^{2}}}}\end{matrix}$and I_(cor) is reflectivity of radiation light with respect to areflection plate.

Assuming a preform model shown in FIG. 11 as heat absorption and heatdiffusion of the preform, it is enough to solve the following equationunder boundary conditions, where R is an outer radius of the preform andr is a distance from a center axis of an aimed position:

-   (Thermal Conductivity Equation)    ${{k{\nabla^{2}T}} + \overset{\bullet}{q}} = {\rho\quad C\frac{\partial T}{\partial t}}$    where T is temperature, t is time, ρ is density, k is thermal    conductivity and or C is specific heat.-   (Heat Absorption←(from radiation heat received by the surface of the    preform))    ${\overset{\bullet}{q} = {\frac{\partial E}{\partial r}}},\quad{E = {E_{0}{\mathbb{e}}^{- {\alpha{({R - r})}}}}}$

That is, the temperature distribution of the respective portions(including thickness direction) of the preform is obtained by solvingthe thermal conductivity equation in consideration of absorption of heatfrom the heater. The absorption of heat from the heater is calculatedfrom radiation heat received by the preform surface. In doing so, theindirect radiation due to reflection is considered in addition to thedirect radiation from the heater. As shown in FIG. 12, since heat of theouter surface of the preform is transmitted to an inner surface of thepreform, intensity of heat transmitted into the preform is obtained bythe radiation heat received by the outer surface of the preform, fromwhich heat absorption within the preform is derived.

By using the preform heating and the heat absorption and heat diffusionof the preform, it is possible to easily perform the thermal analysis.

Industrial Applicability

As described hereinbefore, according to the present invention, it ispossible to perform the design of a shape of a preform and the trialforming of the preform in the fabrication step in the injection blowmolding of a product with high precision to thereby reduce the number offabrication steps, time and cost required for the design and trialfabrication of a preform metal mold. Since, according to the presentinvention, an optimal preform shape in the injection blow molding an bedesigned, it is possible to provide a designing device capable ofreducing a load on a designer thereof.

1. A preform design method comprising a first step of calculating a wallthickness distribution of a mold product on the basis of an initialshape of a preform, a second step of obtaining a difference between thecalculated wall thickness distribution of the mold product and an aimedwall thickness distribution, a third step of comparing the differenceand a threshold value, a fourth step of changing the initial shape ofthe preform on the basis of the difference when the difference exceedsthe threshold value and a fifth step of repeating the first to fourthsteps until the difference becomes smaller than the threshold value. 2.The preform design method as claimed in claim 1, comprising the steps ofpreliminarily setting the aimed wall thickness distribution of the moldproduct, executing a blow simulation by inputting wall thicknessdistribution given at discrete nodes of the preform such that totalweights of the preform and the aimed mold product become coincident,comparing the wall thickness at the nodes of the mold product with theaimed wall thickness at positions corresponding to the nodes, changingthe wall thickness distribution of the preform on the basis of thedifference obtained by the comparison and repeating the preceding stepsuntil the difference of each of the nodes becomes smaller than thethreshold value.
 3. The preform design method as claimed in claim 1 or2, wherein the first step includes a step of calculating the wallthickness distribution of the mold product by using a temperaturedistribution of the preform.
 4. A preform design device comprising meansfor storing an inputted initial shape of a preform as a threedimensional information, first means for calculating a wall thicknessdistribution of a mold product on the basis of the initial shape of thepreform, second means for calculating a difference between thecalculated wall thickness distribution of the mold product and the wallthickness distribution of the aimed mold product, third means forcomparing the difference and a threshold value, fourth means forchanging the wall thickness distribution of the preform on the basis ofthe difference when the difference, exceeds the threshold value andfifth means for repeatedly executing said first to fourth means untilthe difference becomes smaller than the threshold value.
 5. The preformdesign device as claimed in claim 4, wherein said storing meanspreliminarily stores an information of aimed wall thickness distributionof the mold product, said first means includes means for performing ablow simulation by inputting a wall thickness distribution of discretenodes of the preform such that a total weight of the preform becomescoincident with that of the aimed mold product and said second meansincludes means for comparing wall thickness of the mold product thusobtained at the nodes with the aimed wall thickness at positionscorresponding to the nodes.
 6. A program for controlling a computerdevice to execute the respective means as claimed in claim 4 or 5.